With the development of degradation resources, including crude oil or gas containing hydrogen disulfide, line pipes and storage tanks used for transportation and storage of the resources are required to have excellent sour resistance, such as hydrogen induced cracking resistance (hereinafter referred to as a “HIC resistance”), or sulphide stress corrosion cracking resistance (hereinafter referred to as a “SSCC resistance”). Especially, the hydrogen induced cracking is known to be caused in the following mechanism. Specifically, hydrogen entering the steel in a corrosion reaction would be gathered at MnS that is likely to extend in a rolling direction to cause the concentration of stress, or at non-metal inclusions including relatively coarse Nb(C, N) particles left after melting in heating a slab, into gas to cause cracking.
Some techniques for improving the above HIC resistance have been hitherto proposed. For example, Japanese Patent Publication No. 3846233 discloses a steel having improved HIC resistance by controlling an average Mn content and a maximum Mn content of the thickness-directional center of the steel. Such a method can improve the HIC resistance, but does not control inclusions located at a center segregation part, which makes it difficult to prevent fine cracking.
Japanese Unexamined Patent Publication No. 2011-68949 discloses a steel plate which exhibits excellent toughness by appropriately adjusting the amounts of addition of REM and Ca respective to a S content so as to suppress a MnS content. In this way, the addition of REM and Ca can suppress the formation of MnS. However, in order to surely improve the HIC resistance, it is also necessary to appropriately control other sulfides (sulfides of REM and Ca).